This invention relates to phosphate ester functional fluids and more particularly to phosphate ester fluids of improved thermal, hydrolytic and oxidative stability useful as aircraft hydraulic fluids.
Functional fluids have been utilized as electronic coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids, and as a filter media for air-conditioning systems. Hydraulic fluids intended for use in the hydraulic system of aircraft for operating various mechanisms and aircraft control systems must meet stringent functional and use requirements. Among the most important requirements of an aircraft hydraulic fluid is that it be stable against oxidative and hydrolytic degradation at elevated temperatures.
In use, aircraft hydraulic fluids commonly become contaminated with moisture. Water enters the hydraulic system with air bled from an engine compressor stage. During operations, the moisture level in Type IV aircraft hydraulic fluids normally ranges from about 0.2 to about 0.35% by weight. Water causes hydrolytic decomposition of phosphate esters to produce partial esters of phosphoric acid. Hydrolytic breakdown of the ester is accelerated if water content exceeds about 0.5% by weight. Conventionally, phosphate ester aircraft hydraulic fluids are formulated to contain an acid scavenger which neutralizes partial esters of phosphoric acid released by hydrolytic breakdown of the triester. Over time, however, the acid scavenger becomes depleted and organometallic compounds are formed by complex reactions involving the phosphate triester, phosphoric acid partial esters, and surfaces of the metal environment within which the hydraulic fluid is ordinarily contained. These organometallic compounds, of which iron phosphate is usually the most prominent by-product, are not soluble in the hydraulic fluid.
Higher performance aircraft are operated under conditions which expose hydraulic fluids to increasing temperatures. Current Grade A fluids operate at maximum temperatures in the range of 225 to 240.degree. F. However, projected aircraft applications will expose aircraft hydraulic fluids to bulk fluid temperatures in the range of 275.degree. F. or higher. At such temperatures, the potential for oxidative and hydrolytic breakdown of phosphate esters is substantially increased.
Degradation of phosphate ester hydraulic fluids is also accelerated where the fluids are exposed to compressed air. The rate of air oxidation of such fluids also increases with temperature. Thus, for application at 275.degree. F. or higher, a need exists for fluids of both enhanced thermal oxidative stability and enhanced thermal hydrolytic stability.
Erosion problems may also be expected to increase with bulk fluid temperature. Erosion is a form of electrochemical corrosion, more precisely referred to as zeta corrosion, the rates of which are increased with temperature. The incidence of cavitation, which is one of the mechanical sources of erosion problems, is also likely to increase with temperature. As erosion progresses, the presence of metallic or other insoluble components may result in filter clogging and replacement, and can cause a change in the physical and chemical properties of the fluid, thereby requiring premature draining of fluids from the system. Metal contaminants also reduce oxidative stability of the fluid, accelerating corrosion. In addition to any effects resulting from contamination by metal (or other) contaminants, the fluid may suffer deterioration in numerous other ways, including: a) viscosity change; b) increase in acid number; c) increased chemical reactivity; and d) discoloration.
A hydraulic fluid useful in aircraft is available from applicants' assignee under the trademark Skydrol.RTM. LD-4. This composition contains 30 to 35% by weight dibutyl phenyl phosphate, 50 to 60% by weight tributyl phosphate, 5 to 10% of viscosity index improvers, 0.13 to 1% of a diphenyldithioethane copper corrosion inhibitor, 0.005% to about 1% by weight, but preferably 0.0075% to 0.075% of a perfluoroalkylsulfonic acid salt antierosion agent, 4 to 8% by weight of an acid scavenger of the type described in U.S. Pat. No. 3,723,320 and about 1% by weight of 2,6-di-tertiary-butyl-p-cresol as an antioxidant. This composition has proved highly satisfactory in high performance aircraft application. However, it was not designed for extended operations at temperatures in the range of 275.degree. F.